Highly selective etching of silicon dioxide relative to both silicon and resist has been obtained by reactive ion etching substrates which are loaded onto an rf cathode and exposed to a low pressure discharge of a CF4-H2 etching gas mixture. Silicon dioxide-to-silicon etch rate ratios as high as 35 to 1 have been measured and silicon dioxide-to-resist etch rate ratios have been found to exceed 10 to 1. The use of reactive ion etching is important in achieving these high etch ratios; the low operating pressure of between 2.7 and 5.3 Pa and the exposure of substrates to bombardment by energetic ions tend to inhibit polymerization on the substrates. As a result, it is possible to use the greater H2 concentrations which are required for high etch rate ratios. © 1979, The Electrochemical Society, Inc. All rights reserved.